OPENBSD_5_3_BASE denotes the file version of each file comprising OpenBSD-release which becomes officially available in May (as the schedule is today...). No new files will ever be checked into this branch at any time in the future.

OPENBSD_5_3 labels the latest files in the -stable branch. If patches are made to 5.3-release, they will be checked into this branch.

This also means that today's snapshots of -current will be very close to the shipping version of 5.3-release. In the coming days as new snapshots are made available, they will increasingly diverge from the behavior of 5.3-release as development towards OpenBSD 5.4-release begins.